1. Field of the Invention
The present invention relates to a process for the preparation of a magnetic paint, and more particularly to a process for the preparation of a magnetic paint employed for the formation of a magnetic recording layer of a magnetic recording medium.
2. Description of Prior Arts
A magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer provided thereon is used as an audio tape, a video tape, or a recording medium for a computer system. The magnetic recording layer of the magnetic recording medium is formed on a support by coating a magnetic paint containing a binder, a ferromagnetic powder and other additives dispersed in a solvent on the support, and drying the coated layer. For the improvement of the running endurance of a magnetic recording medium, there has been widely employed a powder of high hardness (i.e., abrasive) such as .alpha.-Al.sub.2 O.sub.3 by incorporating it into the magnetic recording layer.
The magnetic paint employable for a magnetic recording layer is generally prepared by dispersing in a solvent various components for the formation of a magnetic recording layer of a recording medium, such as ferromagnetic powder, a binder and an abrasive. That is, in the conventional process for the preparation of a magnetic paint, a ferromagnetic powder and an abrasive are simultaneously dispersed in a solvent.
The present inventors have found that the dispersibility of the abrasive varies depending upon the kind of a binder employed, and the abrasive has a poor dispersibility in the binder comprising a vinyl chloride type copolymer and a urethane resin, particularly in the polyurethane resin. The binder comprising both the vinyl chloride type copolymer and the polyurethane resin having been widely used for improving the running endurance of the resulting recording medium.
Accordingly, if a magnetic paint is prepared by the conventional process comprising simultaneously dispersing the binder, abrasive and ferromagnetic paint in a solvent, the resulting dispersion for a magnetic paint generally contains not a small amount of agglomerates of the abrasive. Such dispersion containing not a small amount of the abrasive agglomerates causes lowering of filtration efficiency when the dispersion is subjected to a filtration step.
Further, some abrasives in the agglomerated state which are not removed by the filtration are introduced into the resulting magnetic recording layer in the same state. The agglomerated particles of the abrasive are liable to drop off from the recording layer, so that the phenomenon of so-called drop-out of reproduction signals occurs because of dropping of the abrasive from the magnetic recording medium.
For preventing the production of abrasive agglomerates in the magnetic paint, it can be employed that the period for dispersing procedure is lengthened whereby enhancing the dispersibility of the abrasive. However, in that case, the ferromagnetic powder in the dispersion is apt not only to be damaged owing to contact with the abrasive so as not to improve the electromagnetic conversion characteristics of the resulting recording medium, but also to become in the exceedingly dispersed state because of the long dispersing time so as to cause reagglomeration of the powder in the magnetic paint.
Japanese Patent Provision Publication No. 56(1981)-167769 describes that the dispersing process is divided into two stages of the first dispersing stage and the second dispersing stage, and the dispersing procedure is performed by mixing a ferromagnetic powder with a portion of a binder in a solvent in the first dispersing stage to prepare a dispersion and further mixing an abrasive and a residual portion of the binder with the dispersion in the second dispersing stage. If a magnetic paint is prepared according to the above-mentioned process, there is given an advantage that the ferromagnetic powder is hardly damaged by contact with an abrasive, because the ferromagnetic powder is brought into contact with the abrasive under condition covered with the binder. However, any improvement cannot be obtained with respect to the dispersibility of an abrasive even by the process described above.